1. Field of the Invention
The present invention relates to an exposure control device for a camera using a shutter whose driving source is a solenoid.
2. Description of the Related Art
In a conventional exposure control device using a solenoid, the solenoid serving as a driving source is driven with such a voltage that is invariable even while a plunger is being attracted or after the plunger has been attracted. When the plunger is attracted, the attraction for the plunger surpasses a repulsive spring force.
FIG. 13 is a graph showing the relationships between the attraction for the plunger of a solenoid used in a conventional exposure control device and the repulsive spring force, and the stroke of the plunger. In FIG. 13, curves Fv1 and Fv2 (drawn in solid lines) represent the characteristics of attractions with solenoid conduction voltages V1 and V2, wherein the voltage V2 is larger than the voltage V1. A curve Fs (drawn in a dashed line) represents the characteristic of the force of a repulsive spring.
In the foregoing conventional exposure control device, the solenoid is driven with the conduction voltage V2 in FIG. 13 during exposure control. When the plunger is repulsed, the attraction assumes a sufficiently large required value relative to the repulsive spring force. When the plunger is attracted, the attraction becomes too large relative to the repulsive spring force.
In the foregoing conventional exposure control device using the solenoid as a driving source, however, when the plunger is attracted, the attraction for the plunger is unnecessarily larger than the repulsive spring force. An excess force is therefore imposed on the plunger. In case the conduction voltage is decreased to the voltage V1, when the plunger is attracted, the attraction for the plunger assumes a sufficient required value. When the plunger is repulsed, however, the attraction is weaker than the repulsive spring force and fails to ensure perfect operation. This kind of problem has arisen in conventional solenoid-driven exposure control devices. In the aforesaid conventional exposure control device, when exposure is performed at a low shutter speed, the heating of the solenoid sometimes damages the driving source. Moreover, excess power is consumed because of the application of a voltage much higher than the voltage required when the plunger is attracted. This poses many problems including that the usable frequency of a battery serving as a power supply decreases.
In an exposure control device disclosed in Japanese Patent Laid-Open No. 4-194912, the repulsive spring force for a plunger is slightly weaker than the attraction for the plunger and has a non-linear characteristic. A high-precision exposure control mechanism is thus realized with a simple mechanism.
A device disclosed in Japanese Utility Model Laid-Open No. 63-54131 uses the forces of two repulsive springs to meet the attraction for a plunger, thus improving the precision in exposure control.
In the disclosures of the unexamined publications, similarly to the aforesaid device, a voltage required for attracting a plunger is applied for a prolonged period of time even after attraction is achieved. The plunger is still subject to damage resulting from heating.